Hi CaptainMattJ
If you have promoted your torpedo, it may be that the extra speed is causing you to miss or just hit the bow. There is a mod to correct this if you dont already have it.
Your method for manual targeting was the same as I used when learning. I found this a good way to get used to keying the info into the TDC.
The next stage for me was learning the Dick oKane method. With this I could forget all about range and focus on methods for getting target speed. (Still use this method alot with TDW AI mod - all torps hitting at same time then escape).
Once I was comfortable with speed I started using the stadimeter.
At the moment I am learning the four bearings method so I can remove the need for any map info. (Kruater has an automated script for this that I find helpful
)